Purdue chemists give an old laboratory bloodhound a sharper nose

WEST LAFAYETTE, Ind.  Purdue University chemists have developed a fast, efficient means of analyzing chemical samples found on surfaces, resulting in a device that could impact everything from airport security to astrobiology to forensic science.

A team, including R. Graham Cooks, has improved the mass spectrometer, a device well known to chemists for its ability to provide information on the composition of unknown substances. Mass spectrometers, essential tools in any modern chemistry lab, are often used by law enforcement to test suspicious-looking residues that could indicate the presence of explosives or drugs inside packages. But while most mass spectrometers are unwieldy, cabinet-sized machines that require samples to undergo hours of intensive preparation before testing, Cooks team has found a way to test untreated samples right where they are found with a mass spectrometer that can fit in a backpack  all by creating a wand that can gather the samples from the environment quickly.

"Weve essentially given an old bloodhound a new nose," said Cooks, who is the Henry Bohn Hass Distinguished Professor of Analytical Chemistry in Purdues School of Science. "While mass spectrometry is one of our best ways to determine the makeup of a substance, the time and effort needed to prepare samples for analysis have made it difficult to use them in the field. With luck, this research will change all that."

The research, which appears in todays (Friday, Oct. 15) issue of the journal Science, was conducted by first author Zoltán Takáts with Justin M. Wiseman, Bogdan Gologan and Cooks, all of Purdue.

Gologan said that the teams innovation was inspired in part by the desire to use one of chemistrys most powerful tools in less limiting environments than the laboratory.

"Testing an unknown sample using standard mass spec can take up to half a day," said Gologan, a graduate student in Cooks laboratory. "You have to dissolve the sample, then dilute the solution, then add additional compounds before you can stick it in the spectrometer. Its a generally effective process, but its not so attractive in situations where time is of the essence."

To simplify these preliminary steps in the process, the team developed a technique known as desorption electrospray ionization (DESI)  a mouthful of a name for a method that is relatively simple to conceive.

"Our device sprays a sample with a stream of high-velocity gas that contains some reactive chemicals," Gologan said. "Just like a strong wind kicks up dust, the gas breaks off a few small particles of the unknown substance, and these combine with the reactive chemicals to form an ionized compound that a mass spectrometer can analyze right then and there. It can be done anywhere, and because there are fewer intervening steps before the substance is analyzed, theres less likelihood that the sample will be accidentally contaminated in the interim."

Sampling is done with a long, tubelike wand that both delivers the gas and sucks up the resulting ionized compound. It is this wand that the team likens to their bloodhounds new nose. The wands tip must come within 5 millimeters of the sample to be effective, but the group has also found a way to build a mass spectrometer that weighs about 18 kilograms (40 pounds), which means it can be carried to the sample, rather than forcing investigators to bring the sample to it.

"This backpack-size device will be useful for field analysis of chemicals, filling a need in airport baggage security and drug detection," said Wiseman, a graduate student working on the project. "While the technique obviously cannot look inside packages to see what is inside, residue from explosives and drugs often remains on the hands of whoever packed it, and some is transferred during handling to the packages surface. That remaining residue is what this device will be good for detecting."

While the team is optimistic about the devices potential for application in the lab and on the street, Gologan cautioned that a better understanding of its functioning was still needed.

"One potential criticism of the device is that we havent yet done an in-depth analysis of the mechanism," he said. "We think we know how it works, but havent yet proven it. Thats one of our next research steps."

Still, Wiseman is optimistic that the teams advancements could have applications far beyond the nations security needs.

"The Mars rovers have concentrated on examining the surfaces of rocks," he said. "Future rovers could use a tool like this to examine other worlds surfaces for the presence of chiral acids, such as the amino acids that form our bodies proteins. It could assist with the search for life elsewhere in the cosmos."

John B. Fenn, who shared the 2002 Nobel Prize for chemistry, said that the DESI technique could also be used for biomedical testing.

"The truly exciting aspect of this work is that substances can be detected on the outer surface of a living species," said Fenn, who is a research professor of chemistry at Virginia Commonwealth University. "Using this technique, the Purdue researchers were able to detect the presence of antihistamine on the skin of a person who had ingested an antihistamine tablet only a short time earlier. The implications of this finding are truly tremendous."

Cooks is associated with several research centers at or affiliated with Purdue, including the Bindley Biosciences Center, the Indiana Instrumentation Institute, Inproteo (formerly the Indiana Proteomics Consortium) and the Center for Sensing Science and Technology. Funding was provided by InProteo. Prosolia Inc., a spin-out subsidiary of InProteo, has rights to commercialize the work.

PHOTO CAPTION:
Postdoctoral researcher Zoltan Takats works with the special "wand" he and a team of chemists developed to improve mass spectrometers, which are powerful  but unwieldy and labor-intensive  tools for chemical analysis. The wand, developed in the laboratory of Graham Cooks at Purdue University, dramatically simplifies and shortens the analysis process, meaning mass spectrometers using the wands could be carried into the field to investigate suspicious residues on packages in post offices and airport baggage rooms. (Purdue News Service photo/David Umberger)

A novel method of desorption ionization is described and applied to the ionization of various compounds including peptides and proteins present on metal, polymer and mineral surfaces. Desorption electrospray ionization (DESI) is carried out by directing electrosprayed charged droplets and ions of solvent onto the surface to be analyzed. The impact of the charged particles on the surface produces gaseous ions of material originally present on the surface. The resulting mass spectra are similar to normal ESI mass spectra as they show mainly singly- or multiply-charged molecular ions of the analytes. The DESI phenomenon was observed both in the case of conductive and insulator surfaces and for compounds ranging from non-polar small molecules like lycopene, the alkaloid coniine, and small drugs, through polar compounds like peptides and proteins. Changes in the solution that is sprayed can be used to selectively ionize particular compounds, including those in biological matrices. In vivo analysis is demonstrated.